What is &:id in Role.all.map(&:id). If (&:id) is shorthand for {|a| a.id}, then how to perform some method over any field individually. For example: Role.all.map(&:id.to_s). It is giving error:
wrong argument type String (expected Proc)
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Marek Lipka
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MEENAKSHI KUMARI
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`Role.all.map(&:id).map(&:to_s)` Although this is both longer and less efficient than `.map{|role| role.id.to_s}`. – Sergio Tulentsev Mar 27 '18 at 09:57
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So &:something is a shortcut to a process. It's the same as a code block being defined after a do, only it can only call a single method on each item in the array.
These three code examples will do effectively the same thing*...
Role.all.map do |role|
role.id
end
In this one, the do and end are replaced with curly braces...
Role.all.map{|r| r.id}
And as we're only calling one method in that block, we can replace it with the shortened syntax you're asking about:
Role.all.map(&:id)
Aside to your question, there is an optimation by which you can get all the ids from Role.
The above block will recall all the rows from your ROLES table, initialize a Role object, then get it's id property. But ActiveRecord provides a method which will just retrieve the id property for every row in the table, and not bother creating a full-fat Role object for each one.
Role.pluck(:id)
This also works for a query on Role...
Role.where("name like '%user'").pluck(:id)
If all you want from the Roles is an id, this is a much more efficient way to get it.
* Sergio disagrees that they are identical because of some subtle differences in run order... You probably don't need to worry about this.
Sergio Tulentsev
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AJFaraday
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do/end and curly braced blocks are not _entirely_ interchangeable. There's a subtle difference in their precedence. – Sergio Tulentsev Mar 27 '18 at 09:59
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@SergioTulentsev I'd have to say that's out of the scope of this question. – AJFaraday Mar 27 '18 at 10:00
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It probably is, but as of now, this answer gives wrong ideas ("exactly the same"). Maybe add a footnote or amend the wording? – Sergio Tulentsev Mar 27 '18 at 10:01
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Btw, you know what I talked about, right? The difference in precedence? – Sergio Tulentsev Mar 27 '18 at 10:06
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@SergioTulentsev I wasn't aware of it, but I have come across it, in similar discussions on the difference between `and` and `&&`. I find it a pretty tedious conversation and have yet to write any code in which it makes any difference. – AJFaraday Mar 27 '18 at 10:07
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Oh, it screws with me almost daily. Like there's a method call `foo do something end`. And I want to debug-print its return value. So I do `p foo do something end`. Bam, it stops working. – Sergio Tulentsev Mar 27 '18 at 10:09
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@SergioTulentsev I'd just put the `p` inside the block... Each to their own, I guess. – AJFaraday Mar 27 '18 at 10:12
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"I'd just put the p inside the block" - not if actual block body consists of multiple lines with branches and such. Alright, I won't annoy you any longer, in this thread. :) – Sergio Tulentsev Mar 27 '18 at 10:13
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